1. Field of the Invention
The present invention relates generally to integrated circuits. More particularly, the present invention relates to an integrated circuit having a lead frame package with internal busses for power distribution and grounding.
2. Discussion of the Related Technology
Integrated circuits are generally mounted in lead frame packages which provide both environmental protection to the semiconductor chip on which the small-scale portion of the integrated circuit is formed, as well as providing for electrical connection of the integrated circuit chip to external circuitry. With large scale integrated circuits having the necessity for a multitude of external electrical connections, the physical limitations of the lead frame package limits the performance of the integrated circuit which may be housed within a particular size of package. This packaging limitation also significantly increases the cost of the integrated circuit by necessitating an increase in the size of the integrated circuit chip itself beyond that which would otherwise be required. This size increase of the integrated circuit chip is necessary simply to allow for the limitations of the packaging technology. In the pertinent art, this limitation imposed by the chip packaging is referred to as an I/O (input/output) limitation on chip performance.
To further elaborate, the lead frame package generally includes a base which is square in plan view and which carries a lead frame. At the periphery of the base this lead frame provides depending legs each forming a separate electrical contact for the integrated circuit. A typical lead frame package might include fifty-two contact legs along each side edge of the package for a total of 208 contacts. The lead frame also includes a plurality of lead fingers each extending from a respective contact leg radially inwardly across the base toward the integrated circuit semiconductor chip. Because the semiconductor circuit chip is small in size compared with the size of the package, the lead fingers taper to become narrower in width and also to become closer together as they approach the integrated circuit chip. In plan view this plurality of lead frame fingers appears like a star burst surrounding the integrated circuit chip.
Such lead frames are generally stamped or chemically etched from sheet material for convenience and economy of manufacture. With a standard lead finger thickness of 0.15 mm, present technology in the production of the lead frames limits the lead finger pitch (center-to-center dimension of the lead fingers) to 0.2 mm. A similar limitation applies to the minimum width of each lead frame finger. Consequently, with each certain number of lead frame contacts and lead frame fingers, there is a technology-limited minimum size of the area bounded by the inner ends of the lead frame fingers and within which the integrated chip itself can be mounted.
Further, the contact pads of the semiconductor integrated circuit chip are generally connected to the inner ends of the lead frame fingers using fine-dimension wire bond conductors. The length of these wire bond conductors are themselves technology limited to a maximum of about 4.5 mm. Consequently, there results a minimum size for the semiconductor chip dependent on the number of contact pads thereon. If the semiconductor chip is made smaller than this minimum size it cannot be connected to external circuitry because of the limitations of the lead frame and wire bond technologies. That is, the lead frame fingers and wire bond conductors cannot reach the small integrated circuit with the large number of such fingers and wire bond conductors required.
As is easily understood, the necessity of making a semiconductor chip larger than would otherwise be required has great cost disadvantages both in terms of material and facilities utilizations. Particularly, an integrated circuit chip which is physically larger than required decreases the number of chips which may be formed on each semiconductor wafer, and decreases the yield of chips from each wafer which is processed through the manufacturing facilities.
Still further, a typical integrated circuit chip may have as many as twenty percent, or more, of its contact pads devoted just to the supplying of power and ground connections to the integrated circuit. Because conventional practice uses one of the lead frame fingers for each one of the contact pads of the integrated circuit chip, a considerable part of the electrical connection capacity of the package is devoted to simply providing power and ground connections to the integrated circuit.